I'm pretty certain that I used Get Mouse Button Down, but I'll double check to be sure.
[EDIT] Yes, I was using GetMouseButtonDown.
However, when I removed action, and even after removing the state, the error remained!
It's very possible that I had a MousePick action earlier on that I removed, but the error didn't get cleared.
When I removed the FSM from the main camera, the error finally went away.
I've now added a new FSM, and GetMouseButtonDown to the starting state, and the error has not reappeared.